Since they were first reported in 1989 Antiferroelectric Liquid Crystals (AFLCs) have been considered very attractive for a number of electro-optic applications, foremost for high-resolution large area displays. In antiferroelectric liquid crystal displays (AFLCDs) the smectic Ca* phase liquid crystal is ideally arranged in a bookshelf geometry, with the smectic layers aligned perpendicular to the cell plates as is illustrated in FIG. 1A. When being in the helix-free surface-stabilized state with the director tilt plane parallel to the cell plates (horizontal tilt plane, HAF configuration) and aligned in so-called bookshelf geometry AFLCs are optically biaxial with their slow principal axis along the smectic layer normal. This gives a dark state when the cell is between crossed polarizers oriented parallel and perpendicular to the layer normal.
When a sufficiently strong electric field ±E is applied across the cell, the anticlinic ground state, with an alternating electric polarization Ps direction in adjacent smectic layers, is forced to one of the two symmetrically situated synclinic states with the slow axis inclined at ±θ away from the layer normal, as is illustrated in FIG. 1B. This is the so-called field-induced transition from the antiferroelectric (AF) to the ferroelectric (±F) state. The switching occurs in domains and greyscale can be produced by controlling the ratio of bright ferroelectric F domains to dark antiferroelectric AF domains. When the field is taken away the AFLC relaxes back to the AF state. The AFLC is thus monostable, in contrast to the surface-stabilized ferroelectric liquid crystal (SSFLC) which is bistable.
So far, the idea to use AFLC materials in a display mode has been in principle, to use three states of the optic axis lying in a plane, and use the hysteretic nature of the electrooptic switching between the AF and the F states. In particular, after the writing of a certain greylevel the application of a holding voltage for which both the AF and the F states are stable, have secured the written greylevel until the next state is to be written. This is the so-called tri-state switching. Small video and large desktop computer prototype AFLC displays have been presented already during the 1990's but they were never commercialized. The main reason for this was the insufficient extinction obtained in the dark state.
AFLC materials are notoriously difficult to align in a high-quality bookshelf structure. Instead AFLC materials typically form an inhomogeneous smectic layer structure with local variations in the slow axis (the effective optic axis) orientation in the cell. These variations in the optic axis orientation cause light leakage in the dark state. One part of the alignment problem is the lack of a nematic phase in AFLC materials. Another is the tendency for the structure to break up under electronic addressing conditions. In the latter situation, the vertical chevron formed at the virgin cooling from the smectic A* to the tilted SmCa*(sometimes via SmC*) is straightened up by the electric field and the AFLC now instead forms a seemingly “horizontal chevron structure”.
In an attempt to solve the “dark state” problem Orthoconic Antiferroelectric Liquid Crystals (OAFLC) have been developed. An OAFLC device features AFLC material which satisfies the orthoconic condition. In order to satisfy the orthoconic condition, two properties are typically met. The first is a material property where the tilt angle in the anticlinic AF structure is approximately 45°, such that the director n in adjacent smectic layers is perpendicular. The second is a device property where the AF structure is surface-stabilized such that no trace of the helix is present. This second condition is harder to realize in the AFLC than in the FLC case.
The result is that if these two conditions are satisfied the AFLC changes from positive biaxial to negative uniaxial and with the optic axis perpendicular to the director tilt plane and to the layer normal instead of being along the normal. Thus, the AFLC may be in an uniaxial negative state with an oblate indicatrix. This is called the orthoconic condition. Thus, the horizontally surface stabilized state (HAF) of orthoconic AFLCs has the optic axis perpendicular to the cell plates. The orthoconic darkstate is in principle just as good as the extinction of the polarizers, independent of alignment or misalignment, which is unique. Between crossed circular polarizers even the bright state is insensitive to alignment, which is equally unique.
So far, grey levels for display applications using AFLC materials have been produced by amplitude modulation using the symmetric hysteresis curve together with a holding voltage.
FIG. 3 is an example of greyscale writing through the use of amplitude modulation with a holding voltage. FIG. 3 illustrates a graphical representation of an applied voltage amplitude with respect to time for each of the four frames 16. FIG. 3 also illustrates a graphical representation of a resulting light transmission for each of the four frames with respect to time 18. FIG. 3 further illustrates examples of respective frame images resulting from the applied voltage 20.
Frame 2, of FIG. 3, is an example of a dark state. As shown, no voltage is applied, thereby resulting in limited or no transmission of light. Frame 3 is an example of a bright state. During a bright state, a constant voltage may be applied allowing for a sufficient transmission of light resulting in the bright state.
Frames 1 and 4 provide examples of greyscale images resulting from amplitude modulation. OAFLC or AFLC amplitude modulation may be provided by using a writing voltage 23 followed by a lower holding voltage 25 to accomplish grey levels with fractional anticlinic to clinic switching. As the applied writing voltage amplitudes for frame one are higher than the writing voltage amplitudes for frame four, the grey scale image of frame four is darker than that of frame one.
By varying the combination of the value of the writing and holding voltage amplitudes, the application time of the writing and holding voltage amplitudes, and/or the difference between the applied writing and holding voltage amplitudes, may other grey levels may be obtained
Grey levels may also be produced by dividing at least one pixel into sub-pixels, as illustrated in FIG. 4. FIG. 4 illustrates an example OAFLC or AFLC display or cell 30 featuring 16 pixels. FIG. 4 further illustrates an example pixel 32, which may feature 3 sub-pixels 32a-32c. By varying the dark and bright states of each of the sub-pixels 32a-32c, vary grey levels may be provided. Such variation may be referred to herein as sub-pixel modulation.
Example pixel configurations 33a-33h are also illustrated in FIG. 4. FIG. 4, further illustrates resulting pixel images 35a-35h, corresponding to respective pixel configurations 33a-33h, which may be provided by varying the dark and bright states of the sub-pixels 32a-32c. Pixel configuration 33a features all three sub-pixels 32a-32c in a dark state, thus the resulting pixel image 35a is a dark state pixel. Similarly, pixel configuration 33h features all three sub-pixels 32a-32c in a bright state, thus the resulting pixel image 35h is a bright state pixel.
Pixel configurations 33b-33g feature sub-pixels in both a dark and bright side, thus resulting in various grey levels. As shown in FIG. 4, the greater the number, or the larger the area, of a sub-pixel in a dark state, the darker the resulting grey level may be. Similarly, the greater the number, or the larger the area, of a sub-pixel in a bright state, the lighter the resulting grey level may be.
It should be appreciated that the sub-pixel system illustrated in FIG. 4 is merely an example. The pixels may feature any number of sub-pixels. Furthermore, any number of pixels in a display system may feature any number of sub-pixels.
To decrease the relaxation time from ±F states to the AF state of an AFLC a kick-back pulse may be used. FIG. 2 illustrates an example of a kick-back pulse. An AFLC device may be addressed with a square pulse to switch to +F or −F state followed by a pulse of reversed polarity which may be shorter and/or of lower absolute amplitude. FIG. 2 illustrates a square pulse 40a followed by a pulse of reversed polarity 40b which comprises a lower absolute amplitude and a shorter time duration as compared to the initial square pulse. The kick-back pulse of FIG. 2 may speed up the back relaxation to the AF state.